Pillow lavas are very common rocks in the Archean. They form when basaltic eruptions occur under water or ice.  They are a significant part of the shields of continental cratons and especially greenstone belts.  Near Marquette they are conspicuous in the Mona Schist along US 41 west of Marquette. It is valuable to use the geometry of pillows to find out what the attitude of rocks is (Which way is “up”?). Not which is up now, but which way was up then.  Pillows seem to give clear up and down information, as pillows that are younger flow over and mold them selves as they fill the gaps between older ones.